1. Field of the Invention:
The present invention relates to a novel process for the synthesis of substituted indolenines having the following general formula ##STR5## in which R is hydrogen, alkyl, aryl cycloalkyl, halogen, or a functional group selected among cyano hydroxyl alkoxy nitro or sulphonic groups, R.sub.1 and R.sub.2 and R.sub.3, are the same or different, and are alkyl, aryl or cycloakyl radicals.
A particularly known substituted indolenine is 2,3,3-trimethylindolenine, which is an important synthesis intermediate in the dyeing and photographic product industry.
For example, starting from 2,3,3-trimethylindolenine or from 1,3,3-trymethyl-2-methylindolenine which is obtained therefrom through methylation and following alkalinization of the reaction medium, it is possible to obtain, by reacting with the suitable reactants, dyes for natural and synthetic fibers containing, as the characteristic chromophore, the methynic group --CH.dbd.CH-- or the azamethynic group --CH.dbd.N--, and various products employable in the photographic field, as dyes, spectral sensitizers and as organic photoconductors in electrophotography.
2. Prior Art
2,3,3-trimethylindolenine up until now has been prepared, analogously to other substituted indolenines, only through the Fisher synthesis, starting from phenylhydrazine and methylisopropylketone, or through a methylation of alkylated indoles, for instance from 2,3-dimethylindole which can be obtained, through the Fisher synthesis, from phenylhydrazine and methylethylketone. Thus, in the past the desired substituted indolenines were prepared using processes which employed very expensive starting materials and difficult procedures.